Radio Frequency Identification (RFID) is a means of using radio frequency (RF) electromagnetic waves to identify objects that carry identifying transponders. This identification technology is similar to bar-code technology. However, instead of using light beams, RFID uses RF electromagnetic waves. Each RFID system consists of at least one RFID reader and, usually, many transponders. During its normal operation, the RFID reader transmits an electromagnetic wave to "illuminate" a target transponder. The transponder responds by selectively reflecting that electromagnetic wave, thereby causing an electromagnetic field disturbance. This field disturbance is interpreted by the RFID reader to reveal the transponder identity and other preprogrammed information stored in the transponder. This process is called back-scatter, selectively reflecting the illuminating electromagnetic waves by changing the energy absorption characteristic of the transponder, thereby creating a field disturbance that can be sensed by the reader's antenna.
There are two main categories of RFID systems: active RFID and passive RFID. Active RFID refers to transponders that are powered by an on-board power source (e.g., a battery, etc.). Passive RFID systems utilize transponders that do not have their own internal power source but rather rely on the transmitted radio waves for self-energization. RFID transponders may have sophisticated designs and usually consist of an antenna, an RFID IC chip, and sometimes an internal or external resonating capacitor. The IC usually stores several kilo-bits of data. Some ICs are read only, some are one-time-programmable (OTP) and some have read/write functions constructed with non-volatile memories such as EEPROM (electronically erasable and programmable read only memory) or FeRAM (Ferromagnetic random access memory). Unlike bar-code labels, RFID transponders are nearly impossible to copy or duplicate. Also unlike bar-code readers, RFID systems can function well in environments containing dust, dirt, grime, oil, snow, darkness, and high humidity. In addition, RFIDs can read or read/write in non-line-of-sight applications, through clothing, wood and nonmetallic materials. These features allow RFIDs to displace bar-code systems in many commercial and industrial applications.
Currently, many models of RFID readers and transponders are made by several different manufacturers. These devices are generally designed to operate in one of four frequency ranges: low frequency (approximately 125 kHz), high frequency (approximately 13.56 MHz), ultra-high frequency (approximately 915 MHz) and microwave (approximately 2450 MHz). Each of these frequency ranges is suitable for different applications. Low frequency readers are used in access control applications, and high frequency readers are used as smart card readers for merchandise source tagging and electronic article surveillance (EAS) applications or electronic money exchange. UHF and microwave readers are used for longer distance and higher data rate asset tracking and asset management applications.
The present invention applies primarily to low frequency 125 kHz access control RFID readers, and primarily for access control applications.
Low frequency RFID readers are basically magnetically coupled devices. When used for access control, the readers are usually installed at the entrances of buildings to control the opening and closing of doors or gates. Typically, short-range (less than 8 inches) readers are for door controls and long-range (greater than 15 inches) readers are for gate controls. During installations, short-range readers are often mounted on drywalls or metal door frames, while long-range readers are often mounted on exterior walls, steel posts or metallic surfaces. Mounting the reader close to a metallic object always degrades the reader's read range. It is not unusual for a short-range reader with a nominal 4-6 inch read range to be mounted on a metal mullion, leaving only a 2-3 inch read range. In the case of long-range readers, the impact is even more severe. Sometimes a 24-inch reader may exhibit less than an 8 inch read range after mounting on a steel column or metallic wall.
A low frequency RFID system relies on reader-to-transponder resonant coupling to transfer energy from the reader to the transponder, and from the transponder back to the reader. Matching the reader's self-resonant output frequency to the reader's excitation frequency is therefore a major factor determining the reader's performance. This frequency match accuracy must be maintained during the production, installation and operation phases of the RFID reader.
Three factors affect the accuracy of the reader's self-resonant circuit: basic component tolerances, consistency during production, and immunity to outside interference during installation and operation of the device.
Controlling component tolerances during production is generally a simple matter of selecting components with tight component tolerances, both initially and at end-of-life. Because RFID readers may be located in hostile environments, choosing components capable of operating reliably and consistently in these environments is also important.
Controlling the self-resonating frequency during production is a different matter. The readers are usually potted to provide protection for the electronic circuits and to provide security. Controlling the potting operation is important because, without careful controls, the potting process itself can affect the self-resonant frequency.
The choice of potting material is important. The dielectric constant of a typical potting compound is generally between 3 to 4, making it quite different from that of air, which is 1. When the potting compound comes in contact with the antenna coil, it increases the inductance of the antenna coil, thereby causing an undesirable downward shift in the resonant frequency. Potting compounds are generally two-part chemicals whose dielectric constant depends on the mixing ratio, the blending process and the cure rate. Since it is hard to control the mixing, blending and curing process, maintaining the consistent self-resonant frequency during potting can be difficult.
A third factor that affects the reader's performance is the installation environment, which is often uncontrollable. Readers may be mounted on drywall, on cement walls, on brick walls and or on metal columns or metal mullions. Since the mounting environment can not be controlled, a means to immunize the reader from the adverse effect of metal mounting becomes necessary.
The last factor that may affect reader performance during operation is the installation process. Most access control readers are connected to an access control panel. This panel has the memory capacity to store all potential identification codes and the capability to interface to a PC or other controller. In addition, the control panel generally includes the system power supply and the necessary circuitry to control door strikes which lock or release the doors. The access control panel is typically located at a central location to control more than one door or gate. This means there may be a significant distance between the reader(s) and the control panel. Potential performance problems may be introduced by long cable runs.
The copper wire size (AWG) that carries the power from the panel to the readers is important. In the RFID reader installation of the prior art, the simplest solution to this problem was to use an oversize wire which generally resulted in both higher material costs and higher labor costs.
It is therefore an object of the invention to provide an RFID reader which operates at a low power consumption and which can tolerate widely varying power supply voltages, with the ability to operate at very low input supply voltages.
It is a further object of the invention to provide an RFID reader which utilizes circuit designs to maximize self-resonant frequency stability.
It is an additional object of the invention to provide an RFID reader manufactured in a manner that also maximizes reader self-resonant frequency stability.
It is another object of the invention to provide an RFID receiver having improved sensitivity to increase the effective operational distance between the receiver and a transponder.
It is a still further object of the invention to provide an RFID reader having metal pre-compensation and post compensation to allow the reader to be mounted directly on a metal surface while retaining most of its read range.
It is yet another object of the invention to provide an RFID reader utilizing a metal compensation technique which shapes the flux field to make it substantially unidirectional.
It is a still further object of the invention to provide an RFID having means for automatically adapting to the interface requirements of an access control system to which it is interfaced.